But automakers and regulators from the EPA, the National Highway Traffic Safety Administration and the California Air Resources Board already are haggling over a tougher proposal initiated by President Barack Obama last October. That plan calls for a CAFE range of 47 mpg to 62 mpg by the 2025 model year.

The key issue: Industry spokespeople say the 62 mpg CAFE, sought by environmentalists, could be too costly and may not be feasible.

One industry ally says hitting 62 mpg would require widespread vehicle electrification, adding nearly $10,000 to the price of a new vehicle. Federal agencies say the cost would be lower -- $3,500 per vehicle, at most -- and would be offset by consumer fuel-pump savings.

After bailing out two of the Detroit 3, federal officials are wary of imposing costs that would depress car sales and cost jobs. Speaking recently in Detroit, EPA official Margo Oge said the agency does not want to hurt auto sales.

"We will be very mindful -- and I underline 'mindful' -- of the consumer throughout this process," Oge, director of the EPA's Office of Transportation and Air Quality, said. "Unless people buy these new clean cars and trucks, and buy them in large numbers, everyone loses."

But the Alliance of Automobile Manufacturers, an industry lobbying group, has warned that the 62-mpg CAFE standard could cut car sales by 25 percent, costing the industry 220,000 jobs.

Oge was scheduled to be in Detroit again last week meeting with automakers on the issue. Her point -- that overly expensive clean technology will do little good -- is also taken up by industry allies. Analyst Sean McAlinden estimates that a 62 mpg bogey will require extensive use of pricey technology such as hybrids.

"That would require a market that's 64 percent plug-in hybrids -- that's the only way we can get it to 62 mpg," said McAlinden, chief economist and executive vice president of the Center for Automotive Research in Ann Arbor, Mich.

New-vehicle prices would rise by an average of $9,970, McAlinden said. Other estimates are lower; the alliance cites a possible price increase of "as much as $6,400." Either way, industry groups say they fear that sticker-shocked consumers would hold on to their old cars.

"That would sort of defeat the purpose of CAFE as well as mandated safety technology, which rely on new-vehicle sales," McAlinden said.

Despite the tug of war, some increase in fuel economy requirements is a certainty.

In September, federal rule makers will propose standards for the 2017-25 model years, followed by a public comment period. The debate over clean technology and its costs will continue until July 2012, when new standards are scheduled to be adopted.

In a recent letter to Sen. Dianne Feinstein, D-Calif., the alliance said "automakers support ... maximum feasible fuel economy standards without negative impacts on affordability, jobs, safety or consumer choice."

The debate is about how aggressive the rules should be. The 62-mpg rule requires a 6 percent annual improvement. An alternate proposal, requiring a 3 percent annual increase, would mandate a CAFE of 47 mpg for 2025. A compromise between the two positions is also possible.

The EPA is taking pains to consult with automakers and is doing extensive research to analyze costs, Oge said in an interview.

The EPA/NHTSA statement adds that with a 62-mpg CAFE, 2025-model vehicles would save $5,700 to $7,400 in lifetime fuel costs. David Friedman, deputy director of the Union of Concerned Scientists' clean vehicle program, said monthly fuel savings could outweigh increased monthly car payments for buyers.

"If you're a typical consumer and you take a five- to seven-year loan, when you drive off the lot, you're saving money," he said.

A key disagreement is over how expensive fuel-saving technology might be with increased volume.

McAlinden cautioned that volume does not always cut costs. Some fuel-saving technologies, such as turbocharging, direct fuel injection and stop-start systems, are being produced at high enough levels that prices already are discounted. Also, he said, the cost of raw materials used in batteries is not likely to drop dramatically.

The two sides also debate the feasibility of getting to 62 mpg with current technology. Sandy Stojkovski, president of AVL Strategic Analytic Services, a technology consulting firm in suburban Detroit, said automakers could meet the less aggressive 3 percent target -- for 47-mpg CAFE -- with advanced internal combustion engines.

But, she said, more aggressive targets will require greater vehicle electrification. Even then, Stojkovski said, the likelihood of creating affordable versions of powertrains such as plug-in hybrids is uncertain.

"Philosophically, you can ask yourself whether you should set regulations based on the possibility that they can do better than that," she said.

Environmental groups counter that hybrids aren't the sole option. Drew Kodjak, executive director of the International Council on Clean Transportation, said a Lotus Engineering study found that automakers could use weight reduction more extensively.

According to the study, automakers could cut vehicle mass, excluding powertrain, by 38 percent for only a 3 percent cost increase. In many cases, the study proposes replacing steel with magnesium, plastic, aluminum and other composites.

The EPA/NHTSA statement says the industry could follow several technological paths to get to higher fuel economy. It says the 3 percent annual increase "requires at most a small increase" in hybrids "and moderate mass reduction on the order of 15 to 18 percent." But it adds that the 6 percent proposal will require a hybrid share of 44 to 68 percent and weight reduction of up to 26 percent.

Eric Fedewa, IHS Automotive director of powertrain and technology, cautioned that automakers hesitate to put fuel economy features on cars unless consumers can see a payback in less than five years. Even then, he said, new features typically take about 13 years to win mainstream acceptance.

"You can't leapfrog," Fedewa said. "You just can't take things from zero to 100 percent, no matter how many incentives you put on them."

"I can say definitively that there are technologies available right now that can get to the 62-mpg level," Kodjak said. "The question is when -- what is a reasonable rate of progress that is appropriate for the industry, and what are the costs associated with it?"

That's the question that the feds, green groups and the industry will hash out over the next 14 months.